This invention relates to a valve driving device of an engine provided with a cam shaft for exclusive use of intake and a cam shaft for exclusive use of exhaust.
An engine with multiple valves, such as disclosed by the Japanese Patent Application Laying Open Gazette No. 62-78453, is known. This engine has three intake valves and two exhaust valves for each cylinder so as to obtain high intake and exhaust efficiency. More particularly, in this engine the three intake valves are arranged on one side of the cylinder bore and are angled toward end portions of valve shafts and are parallel to each other, and the two exhaust valves are arranged on the opposite side of the cylinder bore and are angled toward end portions of valve shafts. Thus, the under surfaces of the valve head parts of the intake and exhaust valves forms a combustion chamber of pent roof shape of high combustion efficiency. According to this engine, the intake valves are driven by a cam shaft for exclusive use of intake valves through the medium of a rocker arm and the exhaust valves are driven by a cam shaft for exclusive use of exhaust valves through the medium of a rocker arm. In the case where a cam shaft is driven in such the engine with multiple valves a transmitting member, such as a timing belt or a timing chain, extends between the cam shaft and the engine output shaft so as to transmit the output torque of the engine output shaft to the cam shaft through the medium of the transmitting member.
In the engine with two cam shafts for exclusive use of intake and exhaust, in case of driving the two cam shafts by the engine output shaft through the medium of the transmitting member, a pulley or sprocket on the driving side is fitted to the engine output shaft and a pulley or sprocket on the driven side, having a diameter about twice in relation to the driving pulley or sprocket on each of the two cam shafts, is required. This requires making the engine larger in size.
From the above, it is suggested to drive either one of the two cam shafts by the engine output shaft through the medium of the transmitting member and to provide both cam shafts with gears which mesh with each other so that the cam shaft driven by the transmitting member in turn drives the other cam shaft, thereby limiting the pulley or sprocket on the driven side to one and making the engine compact.
In the engine with multiple valves (plural valves are provided at least as intake valves or exhaust valves), as shown in FIG. 10, fluctuations of torque applied to one cam shaft according to opening and closing of the valve are large and therefore in case where one of the cam shafts is driven by the other cam shaft through the medium of gears, rotation fluctuation occurs on the cam shaft on the driven side due to torque fluctuation (existence of backlash in gears) and service life of gears shortens due to such torque fluctuations. In this case, if the width of teeth is made larger, it results in checking compactness of the engine.